The present invention broadly relates to generated spiral bevel gears and pertains, more specifically, to a new and improved duplex or double-cut method of manufacturing a generated spiral-toothed bevel gear, particularly the pinion of a bevel-gear or hypoid-gear drive, on a gear-cutting machine.
In its more particular aspects, the present invention specifically relates to a new and improved duplex method of manufacturing a generated spiral-toothed bevel gear on a gear-cutting machine by cutting concave and convex tooth flanks or surfaces by means of cutter heads rotating about respective cutter-head axes and provided with cutters comprising outer and inner blade edges, respectively, whereby there is generated an approximate contact-line tooth bearing or crowning with conjugating tooth flanks of a mating or meshing gear, in that the cutter radii of the outer and inner blade edges and therewith the respective centers of rotation of the cutter heads as well as the center of rotation, i.e. the generating drum axis, of a generating gear are altered in relation to settings or values for manufacturing the mating or meshing gear such that the connecting lines of the centers of rotation of each cutter head and of the generating gear form a parallelogram for the conjugate gear tooth flanks or surfaces of gear and pinion.
The basis for all realized or existing types of bevel-gear tooth systems is the exact toothing or gear-tooth forming with congruent generating gears for generating the pinion and the crown gear, so that pinion and crown gear in the process of meshing fulfil the basic requirement of a gear tooth system in every rolling contact position. Permanent line contact prevails between the meshing or engaged flanks or surfaces. In other words, the crown-gear flank or surface is an exact conjugation of the pinion flank or surface, so that the exact toothing can also be designated as conjugate gear toothing, meaning that the gear ratio is essentially constant during the tooth engagement cycle.
In order to produce congruent generating gears, there is required a geometrical and kinematical adaptation of the gear-cutting machines for the pinion and crown gear. The most significant value or magnitude is thereby the proportional or parallel profile of the tooth depth. There is no connection between the form or profile of the flank line and the position of the generating gears and the congruence of the generating gear flanks, respectively. The manufacture of mating conjugate gear wheels depends solely on the arrangement between the generating drum, the cutter head and the gear blank of the pinion and crown-gear cutting machine.
Exact gear toothings are unsuitable for real or practical use because, for example, oblateness under load conditions, assembly tolerances in the gear box, shaft bearing or mounting systems, toothed-wheel rims and teeth etc., can lead to considerable trouble and malfunction during operation. Therefore, practical toothings comprise flank crowning in order to achieve a localized tooth bearing. There are particularly known elevational tooth bearing, lengthwise crowning and generation crowning as well as higher-order generation-dependent corrections. For instance, elevational tooth bearing is generated by cutter sphericity, while lengthwise crowning is generated by different cutter radii or by inclining the cutter head spindle as disclosed, for example, in Swiss Pat. No. 417,284, published Jan. 31, 1967 of the present assignee, Oerlikon-Buhrle AG, located in Zurich, Switzerland. Generation crowning is generated by kinematic effect. In most cases, there is used a combination of such flank or tooth surface corrections in the manufacturing process of bevel gears with proportional as well as parallel tooth depths. However, localized tooth bearing leads to a basically undesired rotational or kinematic error which depends on the size of the crowning correction.
Known partial generating methods or continuous generating methods for manufacturing spiral-toothed bevel gears having proportional or parallel tooth depths thus relate to gear-cutting machine corrections which ensure the running capability or ability of toothings and, furthermore, make use of the expensive mechanisms to simultaneously optimize the contact behavior as well as to improve the kinamatic conditions during the machining process. It is thereby intended to provide an ideal gearing of wheels which under any possible load ensures a perfect transmission of rotation, i.e. without kinematical error, and possesses a defined tooth bearing or tooth contact displacement behavior dependent solely upon the offset of axes.
In a Technical Memorandum prepared by Faydor L. Litvin et al for the 1985 Off-Highway and Power Plant Congress and Exposition sponsored by the Society of Automotive Engineers, Milwaukee, Wis., Sept. 9-12, 1985, entitled "Generated Spiral Bevel Gears: Optimal Machine-Tool Settings and Tooth Contact Analysis", SAE Technical Paper Series 851573, NASA Technical Memorandum 87075, a method for deriving optimal machine settings for manufacturing generated spiral bevel gears with proportional tooth depths was made known for the first time and according to which a bearing contact or contact-line crowning can be generated without kinematic errors. The suggested corrective measures for improved bearing contact are based on an alteration of the cutter radius and on an imagined parallel and equal displacement of the center of rotation of the generating gear and of the cutter-head axis when the tooth flanks of the pinion are generated in relation to the associated tooth flanks of the mating gear.
However, a variable cutter radius requires a cutter head with continuously adjustable or variable cutters, in order to exactly adjust the required radius of the inner and outer blade edges by distances in the range of approximately 1 mm to 10 mm. This has a negative effect with respect to precision and rigidity. Such cutter heads, in most cases equipped with a reduced number of cutters, have been hitherto fabricated only for laboratory tests or then commercially available cutter heads were specially modified for such laboratory tests. Furthermore, the contact-line crowning or tooth bearing which is a spatial curve requires different machine settings for generating the thrust flanks and the tension flanks and, therefore, precludes the cutting of both tooth flanks at the pinion with one cutter head in one pass or operation and, in other words, precludes the single-cut method or the double-flank cutting method. However, the actually required duplex or double-cut method, in which adjacent flanks, i.e. a thrust flank and a tension flank, are separately cut by respective cutter heads, renders possible highly optimized bevel gear drives, particularly with respect to quiet running and mechanical strength. Moreover, the duplex or double-cut method is at present the only known method for generating correct or pure contact-line crowning or localized tooth bearing.